Anodized coatings are created upon metallic substrates for a great variety of purposes. For example, aluminum aircraft parts often receive an anodized coating for corrosion resistance and for enhanced paint adhesion. Anodize coatings are also used for decorative purposes. Generally, a uniform coating amount or a coating amount within an acceptable range is desired. However, determining uniformity of the coating amount or quantifying the coating amount relative to a desired range may be difficult. Current coating amount testing methods are destructive and therefore cannot be used with final production products. They are also time consuming, environmentally unfriendly, and disruptive to large scale production processes. Anodize coating amount is sometimes specified for some applications and there is no simple non-destructive evaluation for measurement of anodize amount currently known in the art.
Current coating amount testing known in the art is performed by measuring the weight of a coated metallic test specimen. The coating is then chemically removed from the specimen. The metallic substrate is reweighed and the difference is the amount of the anodized coating, which is normally given in milligrams per square foot (mg/ft2). Because the test method is destructive, it cannot be used on the manufactured product. In addition, the currently known testing process only generates a spatially averaged coating amount for the sample. As such, the currently known testing process does not determine coating amount variations over an area.
A notable limitation of the above coating weight method derives from dissimilarity of anodizing conditions between the metallic test specimen and the production parts. For relatively poorly conducting anodize solutions, e.g. phosphoric acid anodize, the part-to-cathode geometry influences the anodize current density achieved. If the metallic test specimen does not encounter “cathode shadowing” while the production part does, then the coating weight attained on the test specimen is not representative of that obtained on the production part.
For some of the thicker anodize coatings, e.g. sulfuric acid anodize and boric-sulfuric acid anodize, nondestructive testing using a conductivity tester can qualitatively indicate that anodize coating has been applied to a metallic substrate. The anodize coating, if present in sufficient thickness, insulates the metallic substrate from the test probes of the continuity tester. This test only shows that sufficient anodize was formed to withstand the mechanical pressure of placing electrical contact probes on the anodize surface. Excessive pressure and slipping at the contact-to-part interface will break through the anodize layer and cause a false indication of inadequate anodize.
For thinner anodize coatings, e.g. phosphoric acid anodizing for structural adhesive bonding, another method of determining whether or not a sufficient amount of anodize coating has been applied to a metallic substrate is by observing color of thin film interference presented by the anodize coating. In the case of phosphoric acid anodizing, birefringence of the anodize coating, that is, a different optical path length depending upon orientation of light polarization, causes the color to change when the polarizer is rotated (cross-polar inspection).
Advances in non-destructive measurement of anodize coating amount have been made. For example, in U.S. patent application Ser. No. 10/171,579, filed Jun. 13, 2002 and entitled “METHOD OF MEASURING ANODIZE COATING AMOUNT USING INFRARED ABSORBANCE” and assigned to The Boeing Company, a method is disclosed for measuring amount of anodize coating using absorbance of a specular component of infrared energy at one wavelength. While this method yields accurate results for specular surfaces, it does not address compensating for scattering of infrared energy from rough surfaces.
Neither visual inspection nor single-frequency infrared absorbance methods address compensation for surface roughness of a substrate underlying an anodize coating. As a result, there is an unmet need in the art for a method for determining an amount of anodize coating on a metallic substrate that compensates for surface roughness of the substrate.